The prior art is replete with refrigeration systems that utilize carbon dioxide as the refrigerant material. Carbon dioxide is ideal for such purposes because its liquid form may be easily flashed to create a refrigerating solid form, commonly known as snow.
The American Frozen Food Institute conducted a feasibility study as to the prospects of developing a cryogenic system suitable for shipping frozen foods and the like in railcars. This feasibility study culminated in an Executive Summary Report, dated March 1985, that described a prototype railcar wherein liquid carbon dioxide was stored in a series of tanks spaced beneath the floor of the railcar. Refrigeration was accomplished by venting the liquid carbon dioxide onto the top of the load stored in the railcar. This venting process formed a blanket of carbon dioxide snow over the load, which was repeated as required during shipment. A drawback of this prototype railcar was that because of the direct contact of the snow with the load, certain products were reduced to extremely low temperatures, thereby becoming very brittle and breaking.
This drawback was circumvented by the design disclosed in Fink et al., U.S. Pat. No. 4,593,536. Fink et al. included a divider that created a bunker along the upper regions of the railcar where carbon dioxide snow was deposited. This bunker system also had the advantage of allowing each railcar to be charged with a load of snow that would last many days, thus alleviating the problem of having to carry a source of liquid carbon dioxide onboard. Vents were provided in the divider along one sidwall that allowed the escape of sublimated carbon dioxide into the storage compartment below to provide the necessary refrigeration. It was theorized that the cold carbon dioxide gas would flow downwardly along the one sidewall, through passageways beneath the floor, and then upwardly along the opposite sidewall and back across the load. In reality, the carbon dioxide gas did not effectively flow upwardly along the opposite sidewall or across the load, thus leaving areas improperly refrigerated during transit.
This drawback was improved upon in the design disclosed in Hill, U.S. Pat. No. 4,704,876. Hill utilized the bunker concept, but provided openings in the divider along both sidewalls, as well as both end walls. Flow of the sublimated carbon dioxide occurred down all four walls until reaching a system of channels located along the floor of the storage compartment. The channels were created by a series of T-beams running substantially the length of the railcar. These channels collected the carbon dioxide gas and routed the gas first to a collection manifold located at one end of the railcar and then to the atmosphere exterior to the railcar through a discharge duct connected to the collection manifold.
An alteration to the basic design of Hill was suggested in Moe, U.S. Pat. No. 4,761,969. It is well known that certain perishable products cannot be allowed to be contacted by carbon dioxide vapors. This is because products such as lettuce, cabbage,asparagus, etc. will turn black or otherwise discolor upon exposure to carbon dioxide vapors, rendering the products aesthetically unappealing to the consumer. In an effort to overcome this problem, Moe disclosed a design that theoretically would allow the refrigerated container to operate in a second mode whereby carbon dioxide snow was created and stored in a flexible bladder located in the bunker. The gases produced upon sublimation of the snow passed to the exterior of the container through a bladder vent, thus keeping the carbon dioxide vapors isolated from the stored product at all times. Under this design, the bladder acted as a cold convection plate to chill the product stored within the lower compartment. To date, this bladder concept has never been commercially employed. Further, it is doubtful that any material could provide the elastic properties required of such a bladder at the tremendously low temperatures associated with using carbon dioxide as a refrigerant.
While both the Hill design and the Moe design provided more uniform refrigeration, the divider between the bunker and the storage area below would often be blown out while the railcar was being charged with liquid carbon dioxide to create the required blanket of snow in the bunker. This problem was due to a number of misconceptions on the part of prior designers. First, it was believed that the blanket of snow would build from the inside out, i.e., from the central region of the bunker beneath the centrally located discharge manifold outwardly to each of the sidewalls. Consequently, the prior designers were not concerned about he vents along the sidewall becoming plugged with carbon dioxide snow. In actuality, just the opposite effect was true. Due to the tremendous pressure at which the liquid carbon dioxide was extruded through the distribution manifold, the blanket of snow would actually build from the outside in, i.e., from the sidewalls inwardly toward the center of the bunker. Thus, plugging the vents was a critical concern. Second, prior designers had not recognized nor accounted for the huge amount of pressure that would build up in the bunker if a proper ventilation area was not provided. This tremendous pressure build up occurred because during the process of converting liquid carbon dioxide into solid snow, only approximately 45% of the liquid carbon dioxide becomes snow. The balance becomes flash gas which must immediately be removed to prevent rupture of the divider.
Thus, the need for a refrigeration system for railcars that allows proper ventilation during the process of creating the blanket of snow in the bunker, and that provides uniform refrigeration during transit, is significant. This invention is directed to satisfying this need.